Can't figure out how to change the "header" (startdate to enddate of week) to display the number of the week instead.

And i tried to figure it out by myself, but all my code did was displaying current week even when i changed week forward/backward <!-- s:-( --><img src="{SMILIES_PATH}/icon_e_sad.gif" alt=":-(" title="Sad" /><!-- s:-( -->

Any ideas how to write the code?
Any help would be apriciated :-)

Best regards
Jonatan, Sweden